This invention relates to a method for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation containing low gravity viscous oils or bitumens. More particularly, this invention relates to recovery of hydrocarbons from tar sands.
The recovery of viscous oils from formations and bitumens from tar sands by conventional methods has generally been unsuccessful because of the high viscosity and low mobility of the oil. While some success has been realized in recovering heavy oils by the use of thermal methods, essentially no success has been realized in recovering bitumens from tar sands. Bitumens can be regarded as highly viscous oils having a gravity in the range of about 5.degree. to 10.degree. API and contained in an essentially unconsilidated sand. These formations containing bitumens are referred to as tar sands. One such deposit is Athabasca tar sand located in Alberta, Canada, which is estimated to contain several hundred billion barrels of oil.
Among the conventional thermal recovery methods applied to produce viscous hydrocarbons from formations and bitumens from the tar sands are steam injection, hot water injection and in-situ combustion. Typically, such thermal techniques employ an injection well and a production well traversing the oil-bearing or tar sand formation.
In a steam operation utilizing two wells, steam is introduced into the formation through the injection well. Upon entering the formation, the heat transferred to the formation by the hot fluid functions to lower the viscosity of oil and improve its mobility while the flow of the hot fluid functions to drive the oil toward the production well from which it is produced.
In the conventional in-situ combustion operation, an oxygen-containing gas such as air is introduced into the formation via a well and combustion of the inplace crude adjacent the wellbore is initiated by one of many accepted means, such as the use of a downhole gas fired heater or a downhole electric heater or chemical means. Thereafter, the injection of the oxygen-containing gas is continued so as to maintain a combustion front which is formed and to drive the front through the formation toward the production well. As the combustion front advances through the formation, a swept area consisting ideally of a clean sand matrix, is created behind the front. Ahead of the advancing front various contiguous zones are built up. These zones may be envisioned as a distillation and cracking zone, a condensation and vaporization zone, an oil bank and a virgin, or unaltered zone.
The temperature of the combustion front is generally in the range of 750-1100.degree.F., which is generated by combustion of coke-like materials deposited on the matrix of the formation. The heat so produced is transferred ahead of the front, where it not only heats the in-place hydrocarbons, but also results in some cracking and distillation of the crude or bitumen.
Ahead of the distillation and cracking zone is a condensation and vaporization zone. This zone is a thermal plateau and its temperature is in the range of from about 200.degree. to about 450.degree.F., depending upon the pressure and the distillation characteristics of the fluid therein. These fluids consist of water and steam and hydrocarbon components of the crude.
Ahead of the condensation and vaporization zone is an oil bank which forms as the in-situ combustion progresses and the crude is displaced toward the production well. This zone of high oil saturation contains not only reservoir fluids, but also condensate, cracked hydrocarbons and gaseous products of combustion which eventually reach the production well from which they are produced.
Various improvements are set forth in the prior art that relate to the injection of water, either simultaneously or intermittently with the oxygen-containing gas to scavenge the residual heat in the formation behind the combustion front, thereby increasing recovery of oil. Prior art also teaches regulating the amount of water injected so as to improve conformance or sweep and to control the combustion.
Despite the use of these thermal recovery techniques none has been particularly successful since long periods of time and considerable amounts of thermal energy are required to heat up a formation sufficiently to obtain the desired reduction in viscosity and improved mobility.
When these thermal techniques have been applied to the tar sands additional problems arise since not only do the tar sands have a low gravity, i.e., 6.degree.-8.degree.API and a higher viscosity, i.e., in the millions of centipoises, but also their permeability is so low that difficulty has been experienced in establishing fluid communication within the formation.
It is an object of the present invention to overcome the difficulties experienced in the application of thermal methods to recovery of heavy oils or bitumens from tar sands, by the use of a low temperature oxidation or controlled combustion method utilizing the injection of an oxygen-containing gas and steam.